#lb, #rb, #lo, #ro {
    border: 1px solid black;
    width: 50%;
    text-align: center;
}
main {
    height: 60%;
    display: flex;
    flex-direction: column;
}
#boven, #onder {
    display: flex;
    flex-direction: row;
    justify-content: space-between;
    
    width: 100%;
    height: 50%;
}